1. Field of the Invention
The present invention relates to the in-process measurement of thin film sample characteristics, and more particularly to self-aligning in-situ ellipsometers used on-line to automatically measure the characteristics of the thin film samples.
2. Description of the Related Art
The manufacture of semiconductor products typically requires the deposition of successive thin film layers on a substrate, which hereinafter will be referred to as a thin film sample. The deposition of thin film layers occurs by passing the substrate through successive vacuum chambers on a production line. At different stages of the manufacturing process, it is often necessary to precisely measure various characteristics of those thin film layers, for example, index of refraction and film thickness. Often those characteristics need to be determined at close to monolayer, that is single atomic or molecular layer, accuracy. To perform those measurements the thin film samples are removed from the production line and taken to a measurement station in a laboratory at which an ellipsometer has been precisely set up and calibrated. The ellipsometer provides data relating to changes in the polarization of light reflected from surfaces of the thin film sample.
The apparatus and methods of using a rotating analyzer in a laboratory to measure the thickness and index of refraction of a thin film on a substrate is well known, see for example, "High Precision Scanning Ellipsometer" by D. E. Aspnes and A. A. Studna and published in Applied Optics, Vol. 14, No. 1, January, 1975. The article describes the determination of the complex reflectance ratios, film thickness, index of refraction and the ellipsometric parameters. In the laboratory, a goniometer, an instrument for precisely measuring angles, is used to determine the angle of incidence of the light beam onto the sample. Using a thin film sample for which the ellipsometric parameters .PSI. and .DELTA. are known, the article describes the collection of light intensity data as a function of the analyzer angle over many analyzer revolutions. The collected data is used to calculate Fourier transform coefficients which in turn are used to calculate the calibration parameters which include an analyzer parameter As, a polarizer parameter Ps, and an attenuation parameter .eta.. Next, using a thin film sample for which the film thickness and index of refraction are unknown, the above process of collecting data and calculating the Fourier transform coefficients is repeated. The complex reflectance ratio .rho. is calculated afterwhich experimental ellipsometric parameters .PSI. and .DELTA. are determined and stored. For purposes of this disclosure, the ellipsometric parameters will always refer to the variables .PSI. and .DELTA..
Next calculated values of the ellipsometric parameters are determined using a models described in the book Ellipsometry and Polarized Light, by Azzam and Bashana, published by North-Holland, 1987. Pages 332-340, "Reflection and Transmission by Isotropic Stratified Planar Structures" describe analysis of reflected light in a multi-film structure. The model is constructed of a series of scattering matrices. For a single layer thin film sample of SiO.sub.2 on a Si substrate, the model is comprised of a first interface matrix I.sub.01 between ambient air and the SiO.sub.2 layer; a layer matrix L.sub.1 comprised of SiO.sub.2 ; and a second interface matrix I.sub.12 between the SiO.sub.2 layer and the substrate of Si. The interfaces are modeled pursuant to the discussion at pages 283-287 of the book, subtitled "Reflection and Transmission by an Ambient-film-substrate System". In the above model, an expression for film phase thickness .beta. utilizes variables representing film thickness d, the film complex index of refraction N, and the angle of incidence .PHI.. In the laboratory setup, the angle of incidence is known and the film thickness is assumed to be equal to the desired value from the manufacturing process. The values of N for air and silicon are well known and used. The real component of the value of N, the index of refraction, for the thin film, for example, SiO.sub.2, is estimated to be its expected value. The imaginary component of the value of N for the thin film layer is assumed to be zero. From the above assumptions, the film phase thickness is determined, and thereafter, the overall complex reflection coefficients and ratios are calculated from which calculated values of the ellipsometric parameters may be determined. An error function, such as a root mean square difference function, is used to compare the experimental and calculated values of the ellipsometric parameters. The above process is repeated for new values of the film thickness and index of refraction of the thin film layer until a minimum for the error function is found. The estimated values of film thickness and index of refraction producing the minimum error function are considered to be the final solution values. It is well known to implement the above models with a computer analysis.
The above well known post process off-line measurement of the characteristics of production samples has the disadvantage of requiring additional manual handling of the production samples. The increased handling adds substantial time to the total processing time and exposes the samples to undesirable contamination.
The off-line process has a further difficulty in aligning the reflection of the light beam from the sample onto the photodetector which measures the intensity of the reflected light beam. Various procedures exist which involve adjusting the light beam or manipulating the sample. Both of those techniques have disadvantages. For example, each time the light beam is adjusted, the angle of incidence is changed and must be measured again. Adjusting the orientation of the sample requires a fixture with mechanisms for changing the orientation of the thin film sample with great precision. Such a fixture is expensive to make and time consuming to use.
Measuring the characteristics of thin film samples while they are on a production line reduces exposure of the samples to contamination and reduces total processing time. However, the samples on the production line are in a vacuum chamber and not readily accessible. Further, automatic handling of the samples on a production line may result in one or more samples having a slightly different orientation which will change the position of the reflected light beam from the sample with respect to the photodetector of the ellipsometer.